DESCRIPTION

These three functions add new static event sources to an event loop. The event loop object
is specified in the event parameter, the event source object is returned in the source
parameter. The event sources are enabled statically and will "fire" when the event loop is
run and the conditions described below are met. The handler function will be passed the
userdata pointer, which may be chosen freely by the caller.
sd_event_add_defer() adds a new event source that will be dispatched instantly, before the
event loop goes to sleep again and waits for new events. By default, the handler will be
called once (SD_EVENT_ONESHOT). Note that if the event source is set to SD_EVENT_ON the
event loop will never go to sleep again, but continuously call the handler, possibly
interleaved with other event sources.
sd_event_add_post() adds a new event source that is run before the event loop will sleep
and wait for new events, but only after at least one other non-post event source was
dispatched. By default, the source is enabled permanently (SD_EVENT_ON). Note that this
event source type will still allow the event loop to go to sleep again, even if set to
SD_EVENT_ON, as long as no other event source is ever triggered.
sd_event_add_exit() adds a new event source that will be dispatched when the event loop is
terminated with sd_event_exit(3).
The sd_event_source_set_enabled(3) function may be used to enable the event source
permanently (SD_EVENT_ON) or to make it fire just once (SD_EVENT_ONESHOT).
If the handler function returns a negative error code, it will be disabled after the
invocation, even if the SD_EVENT_ON mode was requested before.
To destroy an event source object use sd_event_source_unref(3), but note that the event
source is only removed from the event loop when all references to the event source are
dropped. To make sure an event source does not fire anymore, even when there's still a
reference to it kept, consider setting the event source to SD_EVENT_OFF with
sd_event_source_set_enabled(3).
If the second parameter of these functions is passed as NULL no reference to the event
source object is returned. In this case the event source is considered "floating", and
will be destroyed implicitly when the event loop itself is destroyed.

RETURNVALUE

On success, these functions return 0 or a positive integer. On failure, they return a
negative errno-style error code.

ERRORS

Returned errors may indicate the following problems:
-ENOMEM
Not enough memory to allocate an object.
-EINVAL
An invalid argument has been passed.
-ESTALE
The event loop is already terminated.
-ECHILD
The event loop has been created in a different process.

NOTES

These APIs are implemented as a shared library, which can be compiled and linked to with
the libsystemdpkg-config(1) file.